1. Field of the Invention
This invention relates to an automatic coating method and an automatic coating system therefor, using a coating machine particularly suitable for use in coating of vehicle bodies or other articles which involve changes of paint colors.
2. Discussion of the Background
Generally speaking, prior art automatic coating systems are largely constituted by a working mechanism which is located in a predetermined coating area like a coating booth, and a coating machine which is mounted on the working mechanism and provided with a rotary atomizing head to be put in high speed rotation for spraying a paint in the form of finely atomized particles.
The coating machine which are used in the coating system of this sort are largely constituted by an air motor which is mounted within a housing, a hollow cylindrical rotational shaft which is passed axially through the air motor and put in high speed rotation by the air motor, a feed tube which is extended axially and internally through the rotational shaft to supply a paint or a wash fluid, and a rotary atomizing head which is mounted on and at the fore end of the rotational shaft and provided with paint releasing edges on the outer peripheral side thereof.
In this connection, there has been known a feed tube of the construction as described in Japanese Utility Model Laid-Open No. H2-37766. This feed tube is formed in the shape of a double wall tube providing a paint passage internally of an inner tube and a front end washing thinner passage between inner and outer tubes. The paint passage of the feed tube is connected to a color changing valve apparatus, while the thinner passage is connected to a thinner source through a thinner pipe. Thinner is spurted out through the front end washing thinner passage whenever it becomes necessary to wash off deposited paint from the outer periphery of a front end portion of the feed tube. Further, through a drain valve which is constituted by a three-way valve, one end of a waste liquid drain passage is connected to a joint portion of the paint passage of the feed tube with the afore-mentioned paint pipe. The other end of the waste liquid drain passage is connected to a waste tank.
In this instance, the coating machine is mounted on a working mechanism such as a coating robot, reciprocator or the like for a paint coating operation while keeping a predetermined distance from a vehicle body or an object to be coated. At the same time, paint is sprayed by the coating machine toward a coating object (e.g., a vehicle body) for coating the paint thereon.
Namely, while the coating machine is moved by the working mechanism, the rotary atomizing head is put in high speed rotation by the air motor. In this state, a paint from a color changing valve apparatus is supplied to the rotary atomizing head through the paint passage of the feed tube. The supplied paint is released in the form of atomized particles at paint releasing edges of the rotary atomizing head which is kept in high speed rotation. Since a high voltage is applied to the rotary atomizing head, the paint is sprayed as charged particles from the paint releasing edges of the rotary atomizing head, and urged to fly toward and deposit on the coating object under the influence of an electrostatic field which is formed between the rotary atomizing head and the coating object which is connected to earth. The coating system includes controls for the coating machine and the working mechanism to carry out a coating operation on the coating object automatically.
When changing the paint color, a previous color which remains in the paint pipe is washed away before supplying a fresh color thereto. In a washing stage, the drain valve is opened in the first place to secure a route from the paint feed pipe to the waste tank through the waste liquid drain passage. Then, thinner and air are alternately supplied from the color changing valve apparatus to clean the paint pipe.
In the next place, thinner is supplied again from the color changing valve apparatus to wash the paint passage in the feed tube and the rotary atomizing head. This thinner is flushed toward the rotary atomizing head through the paint pipe and the feed tube, thereby washing the paint passage as well as the rotary atomizing head.
Further, from a front washing thinner source which is provided separately from the color changing valve apparatus, thinner is supplied to the outer periphery of a fore end portion of the feed tube via thinner pipe, front washing valve and front washing thinner passage of the feed tube to wash spot-wise the outer periphery of a fore end portion of the feed tube. Thereafter, paint of a fresh color is supplied to the paint passage in the feed tube from the color changing valve apparatus in preparation for a coating operation for the new color.
Thus, according to the paint sprayer machine which is disclosed in Japanese Utility Model Laid-Open No. H2-37766, the paint pipe between the rotary atomizing head and the color changing valve apparatus as well as the rotary atomizing head and the outer periphery of a fore end portion of the feed tube have to be washed each time before changing the paint color. A washing operation of this sort not only takes a long time but also invites a conspicuous increase in running cost due to large consumption of thinner or wash fluid.
Attempts have been made to develop a coating machine which can overcome these problems, as disclosed in Japanese Patent Laid-Open No. H6-134354 and H6-269702. The prior art coating machines described in these publications are of the type which uses a bundle of feed tubes for feeding various paint colors separately from the respective paint sources through the respective paint pipes, paint valves and paint feed tubes.
In the case of the coating machines of the bundled feed tube type, having a bundle of feed tubes including a plural number of paint feed tubes for different paint colors and a wash feed tube which is connected to a thinner source through a thinner feed pipe having a wash valve connected within the length thereof.
According to the prior art coating machines of the type just mentioned, the each one of the bundled paint feed tubes is connected to a paint supply source of a specific color through a paint feed pipe and a paint valve which is provided within the length of the paint feed pipe. The paint which is supplied to a rotary atomizing head through one of the bundled feed tubes is released in the from of atomized paint particles from the rotary atomizing head which is put in high speed rotation. Since a high voltage is applied to the rotary atomizing head, the released paint particles are thereby charged and caused to fly toward and deposit on a coating object which is connected to earth.
On the other hand, when changing the paint color, a wash fluid like thinner is fed to the rotary atomizing head through a wash feed tube for washing away blots of previous color from the rotary atomizing head.
In the case of the coating machine as described in the above-mentioned Japanese Utility Model Laid-Open No. H2-37766 having a double-wall feed tube, there is an inherent problem that the machine construction is complicated due to the necessity for connecting the washing thinner passage of the feed tube to a washing thinner source through a thinner pipe separately from the color changing valve apparatus.
Similarly, the machine construction is complicated in the coating machines disclosed in Japanese Patent Laid-Open No. H6-134354 and H6-269702 which require to connect a wash feed tube in the bundle of feed tubes to a thinner source through a thinner feed pipe separately from other paint feed tubes which are allocated to different colors.
Further, each one of the above-mentioned prior art coating machines has a problem in that, when washing a rotary atomizing head, a coating area is contaminated considerably by paint and thinner which are scattered around when released from the rotary atomizing head.
Furthermore, in any of the above-mentioned prior art coating machines having a wash thinner passage (or a wash feed tube) located on a coating machine body, there is a possibility of an applied high voltage flowing toward the thinner source through the thinner in the wash thinner passage (or a wash feed tube). Therefore, the thinner to be used for a washing operation is required to maintain a certain level of electrical resistivity and in a large quantity because it cannot be selected from inexpensive products simply in consideration of washing capacity.